1. Field
The present invention relates generally to magnetoresistive devices and, more particularly, to giant magnetoresistive devices for magnetic random access memory applications.
2. Related Art
The discovery of the giant magnetoresistive (GMR) effect has led to the development of a number of spin-based electronic devices. The GMR effect is observed in certain thin-film devices that are made up of alternating ferromagnetic and nonmagnetic layers. The resistance of a GMR device is typically lowest when the magnetic moments of the ferromagnetic layers are in a parallel orientation and highest when the magnetic moments are in an antiparallel orientation.
One type of GMR device is commonly referred to as a “spin valve.” A spin valve typically includes two ferromagnetic layers that are separated by a thin layer of a non-magnetic metal (usually copper) and also includes an antiferromagnetic layer that “pins” the magnetization of one of the ferromagnetic layers. FIG. 1 illustrates (in a simplified form) the layers in a typical spin valve 10. As shown in FIG. 1, spin valve 10 includes ferromagnetic layers 12 and 14 separated by a nonmagnetic layer 16. Ferromagnetic layer 14 is adjacent to an anti-ferromagnetic layer 18, such that the magnetization of ferromagnetic layer 14 is “pinned” in a particular orientation. The arrow in layer 14 indicates an exemplary pinned orientation, though, in general, the orientation could be pinned in either direction. Thus, the magnetization of ferromagnetic layer 14 remains relatively fixed when moderate magnetic fields are applied to spin valve 10. In contrast, the magnetization of ferromagnetic layer 12 is free to switch between parallel and antiparallel orientations, as indicated by the double-arrow symbol in layer 12. Thus, by applying an appropriate magnetic field to spin valve 10, the magnetization of ferromagnetic layer 12 can be changed while the magnetization of ferromagnetic layer 14 remains the same. In this way, applied magnetic fields can change the relative orientations of the magnetizations in ferromagnetic layers 12 and 14, which, in turn, can be detected as a change in resistance. In particular, the resistance of spin valve 10 is typically lowest when the magnetizations of ferromagnetic layers 12 and 14 are parallel and highest when the magnetizations are antiparallel.
Another type of GMR device is commonly referred to as a “pseudo spin valve.” Like a spin valve, a pseudo spin valve typically includes two ferromagnetic layers that are separated by a layer of a nonmagnetic metal, with the magnetization of one of the ferromagnetic layers staying relatively fixed when moderate magnetic fields are applied. However, in a pseudo spin valve, this fixed magnetization is a result of a relatively high anisotropy and switching field rather than a result of being pinned. FIG. 2 illustrates (in a simplified form) the layers in a typical pseudo spin valve 20. As shown in FIG. 2, pseudo spin valve 20 includes ferromagnetic layers 22 and 24 separated by a non-magnetic layer 26. Ferromagnetic layer 24 has a relatively high anisotropy and switching field, so that its magnetization remains relatively fixed when moderate magnetic fields are applied to pseudo spin valve 20, as indicated by the arrow symbol in layer 24. In contrast, ferromagnetic layer 22 has a lower anisotropy and switching field, which, in many cases, is achieved by making ferromagnetic layer 24 substantially thicker than ferromagnetic layer 22. As a result, the magnetization of ferromagnetic layer 22 is free to switch between parallel and antiparallel orientations, as indicated by the double-arrow symbol in layer 22. Thus, by applying an appropriate magnetic field to pseudo spin valve 20, the magnetization of ferromagnetic layer 22 can be changed while the magnetization of ferromagnetic layer 24 remains the same. The resistance of pseudo spin valve 20 is typically lowest when the magnetizations of ferromagnetic layers 22 and 24 are parallel and highest when the magnetizations are anti-parallel.
GMR devices, including spin valves and pseudo spin valves, can be used as data storage elements in magnetic random access memory (MRAM) devices. In this regard, exemplary MRAM applications of GMR devices are described in U.S. Pat. Nos. 6,147,922; 6,175,525; 6,178,111; and 6,493,258, all of which are incorporated herein by reference. In typical MRAM devices, the logical state of a GMR-based memory element is based on its resistance, which, in turn, is based on the relative orientations of the magnetizations of the ferromagnetic layers. Thus, in one logical state, e.g., a “0” state, a GMR device may have its ferromagnetic layers in a parallel orientation and, thus, may exhibit a low electrical resistance. In the other logical state, e.g., a “1” state, the GMR device may its ferromagnetic layers in an antiparallel orientation and, thus, may exhibit a higher electrical resistance. Data may be written to a GMR-based memory element by applying a magnetic field sufficient to change the magnetization of the “free” ferromagnetic layer, i.e., ferromagnetic layer 12 in spin valve 10 or ferromagnetic layer 22 in pseudo spin valve 20. In this way, the “free” ferromagnetic layer functions as a “switching layer” that stores data in the form of a particular magnetization orientation relative to the other ferromagnetic layer, the “reference layer.” Thus, in spin valve 10, ferromagnetic layer 12 may function as the switching layer, and ferromagnetic layer 14 may function as the reference layer. Similarly, in pseudo spin valve 20, ferromagnetic layer 22 may function as the switching layer, and ferromagnetic layer 24 may function as the reference layer.
The magnetic fields used to write data to a GMR-based memory element in an MRAM device are typically generated by a “word” current flowing in a nearby conductor. For example, a word current flowing in one direction may be used to place the GMR-based memory element in one logical state, and a word current flowing in the other direction may be used to place the GMR-based memory element in the other logical state. In particular, in some of the common MRAM architectures, each memory element includes two GMR-devices that are in opposite logical states. Thus, to change the state of the two GRM devices in the memory element, the word current is often arranged to apply magnetic fields of the same magnitude but opposite sign to the two GMR devices. Because of magnetic hysteresis, the switching layer may retain its magnetization orientation relative to the reference layer even when the word current stops and the magnetic field that the current generated is no longer present. In this way, little or no power may be needed in order for a GMR-based memory element to retain its logical state. Accordingly, MRAM devices are generally regarded to be a form of non-volatile data storage.
One difficulty with conventional GMR devices for MRAM applications, e.g., for write and/or read operations, depending on the architecture, is that the hysteresis curve for a GMR device is often substantially “biased,” i.e., asymmetric with respect to applied magnetic field. FIG. 3 illustrates such a biased or asymmetric hysteresis curve. In FIG. 3, the vertical axis represents the resistance of an exemplary GMR device, and the horizontal axis represents applied magnetic field. This GMR device exhibits a resistance R1 in zero applied magnetic field, after a magnetic field H1 is applied, and exhibits a resistance R0 in zero applied magnetic field, after a magnetic field H0 is applied. Thus, R1 may represent the resistance of the GMR device in the “1” state, and R0 may represent the resistance of the GMR in the “0” state.
Several disadvantages may result from this asymmetric hysteresis curve. First, because the hysteresis curve is not centered about zero applied magnetic field, the difference between the two zero-field resistances, R1 and R0, may be much smaller than the maximum possible resistance difference possible that the GMR device can exhibit. Second, the asymmetry of the hysteresis curve may cause higher word currents to be required for reliable operation. In particular, since word currents of the same magnitude but different directions are typically used to write data to the GMR devices in an MRAM memory element, a word current that generates an applied magnetic field with magnitude H0 may be insufficient. Flowing in one direction, the word current may be able to place the GMR device in the “0” state with resistance R0. However, when flowing in the other direction, the word current may be unable to place the GMR device in the “1” state with resistance R1. Instead, a higher word current, sufficient to generate an applied magnetic field of magnitude H1 may be required for reliable operation.
Accordingly, there is a need for GMR devices that exhibit hysteresis characteristics that are more compatible with MRAM applications.